This invention concerns a portable power grinder of the type including a housing, a rotation motor with a rotor shaft having a rear end portion extending out of the motor, an output shaft drivingly coupled to the motor rotor and intended to carry a grinding tool, and a speed governor for providing a constant motor speed.
A serious problem to deal with at in rotating power grinders is the risk for accidentally reaching overspeed and a resulting fast disintegration or blowing up of the grinding wheel. This may occur as a result of malfunction of the speed governor of the grinding tool or, perhaps more commonly, as a result of using a grinding wheel intended for a much lower maximum speed.
One reason why an improper grinding wheel may be mounted on the output shaft is that the maximum speed marking on the tool housing is incorrect. The marking may not inform of the maximum speed level provided by the speed limiting means actually mounted in the tool. Most suppliers of power grinding tools offer a number of different speed limiting means fitting into the same tool housing but providing different maximum speed levels, and if a tool is assembled with a speed limiting means which provides a higher maximum speed level than what is informed of by the tool housing marking, there is a great risk that the grinding wheel attached to the output shaft will not withstand the inertia forces generated at this higher speed level.
It is an object of the invention to create a solution to the above problem by providing an improved power grinder design which ensures that the maximum speed level provided by the speed limiting means does not exceed the speed level informed of by the tool housing marking.
A previously known power grinder having a speed marking which safely corresponds to the speed limiting means actually fitted in the tool housing is shown in U.S. Pat. No. 4,444,272. In this prior art tool, the speed marking is located on top of a non-rotating cup-shaped member forming an integral part of the speed governor and overspeed safety device of the tool. This cup-shaped member is exposed through an aperture in the tool housing such that the speed marking is readable from outside the housing.
This known speed marking arrangement, however, is restricted in its applicability to power grinders having fly-weight actuated speed governors and overspeed safety devices which are mounted at the rear end of the motor shaft.
This restricted applicability is a drawback which is avoided by a power grinder according to the invention.
Other features and advantages of the invention appear from the following specification and claims.